param

to extract from

returns

[[String]] or number

example

import {F} from 'ts-toolbelt'

const fn = (a1: any, a2: any) => {}

type test0 = F.LengthOf<typeof fn>               // 2

type test1 = F.LengthOf<(a1?: any) => any>       // 0 | 1

type test2 = F.LengthOf<(...a: any[]) => any>    // number